Some wavelength-division-multiplexing-passive-optical-networks require precise wavelength alignment between the wavelengths of the signal from a transmitter in a central office to a device in a remote site distributing that signal to a subscriber. In a passive-optical-network, a remote node containing the signal-distributing device is typically located outdoors without any electrical power supply. The transmission wavelength of the outdoor signal-distributing device can change according to the variation of the external temperature. Misalignment of the wavelength between the transmitted signal and the operating wavelength of the device distributing the signal introduces extra insertion loss in the signal.
A possible way to minimize the misalignment can be to use a narrow-linewidth distributed feedback laser diode (DFB LD) as an optical transmitter to satisfy the wavelength alignment condition. However, this arrangement may not be an economic solution because of the high price of each DFB LD.
Another passive optical network may use a broadband light emitting diode (LED) as an optical transmitter. However, the modulation bandwidth of the LED can be narrow, thereby, making it difficult to send data at a high bit rate. Moreover, long-distance transmission in a passive optical network can be difficult with an LED due to the inherent weak power output from an LED.
Complex channel selection and temperature control circuits have been employed to compensate for the large insertion loss in optical signals passing through optical multiplexer/demultiplexers located in different locations. The operating wavelength of these devices can vary depending on the temperature of the device. However, the complexity of the channel selection circuit has the disadvantage that the complexity the circuit becomes greater and greater as the number of input ports of the circuit increases. Thus, the more channels being distributed by a multiplexer/demultiplexer, then the more complex and expensive the channel selection and temperature control circuit becomes.